Metering devices of the type indicated above are known and generally comprise a substantially cylindrical metering chamber with an outlet opening for the fluid or resin to be metered, a gripping handle which can be fixed to the metering chamber, and a rod with a piston housed in the metering chamber in order to expel the fluid through the outlet opening. The piston is located at a longitudinal end of a rod which has at its opposite end an operating member which may be actuated manually by a user.
In order to allow accurate metering under pressure, especially where the fluid is in paste form or has a high viscosity, screw-type actuating means are envisaged and essentially consist of a thread which is formed on the outer surface of the rod with the piston and is intended to co-operate a female thread associated with the metering chamber. By imparting a rotation to the threaded rod by means of the operating member, the piston is made to move along the metering chamber causing the resin to be expelled in a controlled manner.
An example of a metering device of the type mentioned above is illustrated in German application DE-A-3,443,167. A drawback of said known device consists in the fact that, in order to fill the metering chamber in situ with the resin or bone cement to be metered, it is necessary to rotate the threaded rod in the opposite direction to that of delivery, which requires a certain time during which the resin may harden at least partially, reducing its fluidity and plasticity.
In order to prevent this problem, a number of manual metering devices of the type illustrated in European patent application EP-A-1,054,231 and French application FR-A-2,690,332 have been proposed, in which the metering chamber is provided with a lateral opening—which is different from the outlet opening—for the introduction of the fluid to be metered into the chamber.
The opening for introducing the resin is connected by means of a connecting line to a storage tank or an auxiliary syringe for introducing the fluid, which must necessarily be loaded beforehand in one of said auxiliary containers.
A drawback of said known former solution consists in its relative complexity and dimensions. Moreover, the procedure of transferring the fluid into the chamber is fairly long and elaborate and may lead to the partial hardening of said resin.
Dispensers of plastic fluids, such as gypsum or mastics, are known from U.S. Pat. No. 4,485,944 and U.S. Pat. No. 5,253,589, in which it is possible to change over from actuation of the piston with screw means to direct longitudinal actuation without screw means.
Another device of this type, which has the specific function of performing vertebral plastic surgery operations, is described in European patent application EP-A-1,157,677 and comprises a piston which may be actuated either with a screw mechanism or by means of a simple longitudinal movement without a screw mechanism. In particular, the female thread is formed by teeth which engage with the threaded rod in such a way as to transform the rotary movement of the threaded rod into the translatory movement of the piston. However, by moving the teeth away from the threaded rod, this rod may be freely moved axially so as to exert direct pressure on the bone cement. For this purpose, the teeth may slide inside the gripping handle against the opposing action of a spring and may be moved away from and towards the threaded rod by operating a lateral slider which protrudes from the handle.
Owing to this configuration, after operating the slider in order to move the teeth away from the threaded rod, the bone cement may be pressurised inside the metering chamber, both during intake and during delivery, as occurs with a normal syringe. At this point, after bringing the teeth towards the threaded rod by operating the slider, it is possible to perform micrometric feeding of the piston by rotating the operating member.
The main drawback of this known device consists in the significant constructional complexity of the mechanism for actuating the teeth, as well as the large number of component parts, resulting in the consequent high cost of the device as a whole.
This significant cost makes it disadvantageous from a cost-related point of view to use the device once only and therefore results in the need for the device to be cleaned and sterilised in an autoclave or applying other methods every time it is used, with the resultant need to use materials which are of high quality and/or resistant to heat and sterilisation procedures.
Another and perhaps greater drawback consists in the fact that the lateral slider must be moved with one finger of the hand while the other fingers grasp the handle and this makes the operation of disengaging the teeth difficult, especially where fluids and moisture are present in the operating area.
It is therefore possible that the doctor or surgeon, whose hands are normally covered in blood and other body fluids, will be unable to operate the slider easily and will be forced to ask for help or to use the other hand, in circumstances where it is necessary to act rapidly, also because the cement hardens in just a few minutes.
Last but not least another drawback consists in the fact that said known device has a slider on just one side, in particular on the left side, so as to allow the threaded rod to be actuated with the right hand, and therefore cannot be easily used by left-handed people.
It would be convenient to have a metering device which is more functional and can be handled easily compared to known metering devices in all conditions of use and by any user.
Moreover, it would be desirable to have a metering device which has a greater constructional simplicity so as to reduce its production cost considerably. In this way, after its initial use in conditions of maximum hygiene and safety, the metering device could be thrown away, avoiding laborious and costly cleaning and sterilisation operations.